Deleting an object in Ruby

Question

Let's say I have the following class:

class Vehicle
    @@total_vehicles = 0
    @@all_instances = Array.new

    def initialize
        @@total_vehicles += 1
        @@all_instances << self
    end

    def total_vehicles #returns total number of Vehicles 'alive'
        return @@total_vehicles
    end

    def all_vehicles #returns an array of all Vehicle objects
        return @@all_instances
    end

end

Now to keep @@total_vehicles and @@all_instances up-to-date and correct, I want to make sure that they are correctly decremented and updated, respectively, when one of those objects is garbage collected. But here is what happens:

v = Vehicle.new
Vehicle.total_vehicles # => 1
v = nil #no references to Vehicle instance now
ObjectSpace.garbage_collect #instance garbage collected
Vehicle.total_vehicles # => 1    Nope!

Well I could add a finalizer Proc to each instance of the Vehicle class that, when called upon the object's garbage collection, would be called. But according to the documentation, ObjectSpace.define_finalizer(v,someProc) would call someProc after the Vehicle instance is destroyed - meaning I cannot use self or self.class in there (since there would be no class, as there is no object!) I could have the proc call a public accessor method on the Vehicle class, but that takes away the purpose of class variables being accessible only to the class and its instances -> essentially turning the class variables into gvars.

How can I have the equivalent of a destructor method (from C++) that will get a Vehicle instance's affairs in order, as it were, before getting garbage-collected?

P.S. ObjectSpace#count_objects is no a viable option, as even the Ruby docs are up front about.

Answer 1

Right now, they will never be garbage collected, as you are holding a reference in @@all_instances. You could use a finalizer to get the result you want:

class Vehicle
  class << self
    attr_accessor :count
    def finalize(id)
      @count -= 1
    end

    def all #returns an array of all Vehicle objects
      ObjectSpace.each_object(Vehicle).to_a
    end
  end
  Vehicle.count ||= 0

  def initialize
    Vehicle.count += 1
    ObjectSpace.define_finalizer(self, Vehicle.method(:finalize))
  end
end

100.times{Vehicle.new}
p Vehicle.count  # => 100
ObjectSpace.garbage_collect
p Vehicle.count  # => 1, not sure why
p Vehicle.all    # => [#<Vehicle:0x0000010208e730>]

If you run this code, you will see that it "works", except that there remains one Vehicle that is not garbage collected. I'm not sure why that is.

Your count method could be also defined more simply by returning ObjectSpace.each_object(Vehicle).count

Finally, if you really want to maintain a list of existing Vehicles, you need to store their ID and use ObjectSpace._id2ref:

require 'set'

class Vehicle
  class << self
    def finalize(id)
      @ids.delete(id)
    end

    def register(obj)
      @ids ||= Set.new
      @ids << obj.object_id
      ObjectSpace.define_finalizer(obj, method(:finalize))
    end

    def all #returns an array of all Vehicle objects
      @ids.map{|id| ObjectSpace._id2ref(id)}
    end

    def count
      @ids.size
    end
  end

  def initialize
    Vehicle.register(self)
  end
end